Opioids such as morphine are the most powerful treatment for pain. Unfortunately, the analgesic effects of morphine decrease with repeated administration because of tolerance. Many mechanisms have been proposed to underlie the development of tolerance. Recent studies suggest that opioid binding at the mu-opioid receptor may be a key step in this process. Administration of high efficacy mu-opioid receptor agonists produce maximal receptor signaling, rapid desensitization of the mu-opioid receptor, and receptor internalization, but relatively little tolerance. In contrast, morphine produces minimal desensitization and receptor internalization, but tolerance is rapid and pronounced. Although these findings suggest that agonist efficacy and mu-opioid receptor internalization are important factors in tolerance to opioids, most of these data are derived from in vitro studies using brain slices or cultured cells lines. The objective of the proposed studies is to determine whether the knowledge gathered using these reduced preparations apply to tolerance mediated by the periaqueductal gray (PAG) in intact rats. In particular, the proposed studies will test the hypothesis that changes in mu-opioid receptor signaling in the PAG causes tolerance to the anti nociceptive effects of opioids. This hypotheSis will be tested by determining whether mu-opioid receptor internalization contributes to tolerance to morphine microinjections into the PAG. The strength of these studies lies in correlating physiological and anatomical changes in PAG neurons and behavioral measures of tolerance to the anti nociceptive effects of morphine. An understanding of the mechanisms underlying tolerance in intact rats will allow the development of better treatments for chronic pain patients who are tolerant to the analgesic effects of opioids.